Conventional ink jet printing ink compositions contain large amounts of solvents leading to the disadvantage that the substrate to be coated must be resistant to the used solvent. Moreover, conventional coatings are usually dried by applying heat in order to remove the solvent and to cross-link the binding agents of the ink jet printing ink composition. If the substrate is heat-sensitive, the drying has to be performed at a low temperature for a longer period of time.
In view of that, UV ink jet printing ink compositions are advantageous in that these compositions do not contain solvents or only minor amounts of solvents and are dried or cured by irradiation of UV light.
Particular UV ink jet printing ink compositions are photo curable ink compositions for ink jet recording. For these photo curable ink compositions, it is necessary that the ink printed on the surface of a substrate, such as a sheet of paper or a foil, is quickly cured so that the printing process can be performed at a high velocity.
US 2010/0036036 A1 discloses a photo curable ink composition for ink jet recording comprising a metallic pigment, a polymerizable compound, and a polymerization initiator, wherein the metallic pigment is a plate-like particle composed of a laminate, including a metal or alloy layer, and at least one selected from a resin layer and a silicon oxide layer.
The polymerizable compound includes monofunctional polymerizable compounds and polyfunctional polymerizable compounds. According to the teaching of US 2010/0036036 A1, these polymerizable compounds are not particularly limited as long as polymerization is initiated by being supplied with any kind of energy to cure, and every form of a monomer or an oligomer, a linear polymer and a dendritic polymer can be used.
US 2007/0249750 A1 discloses a two-component photo curable ink composition set comprising an ink composition A containing at least a photoradical polymerization initiator and a radically polymerizable compound and not containing a colorant; and an ink composition B containing at least a colorant and a radically polymerizable compound and not containing a photo radical polymerization initiator.
EP 1 914 279 A2 discloses another two-component photo curable ink composition set comprising an ink composition A that includes at least a color material, a polymerizable compound, and a radical polymerization photoinitiator; and an ink composition B that includes at least a radical polymerizable compound and does not include the color material and the radical polymerization photoinitiator, wherein the ink composition A and/or B includes a sensitizer.
A two-component photo curable ink composition is disadvantageous in that a user has to mix the two parts in a specified ratio and then to use up the mixed two-component photo curable ink composition in a given period of time.
Especially in ink jet printing inks the use of a two-component photo curable ink is impractical. Most ink jet printing inks are delivered to the customer in a closed tank that cannot be opened. Therefore the customer does not have the chance to mix the two components. In case the two components can be mixed, the mixed ink must be stable in terms of not starting polymerisation before the printing process has occurred. This is a quite demanding task as a UV curing process in ink jet printers usually involves high energy irradiated by a lamp. This energy leads to a temperature increase of the inks stored in the respective tanks leading to increased danger of undesired polymerisation.
Another possibility would be to use two separate tanks for each component to be stored. Cumbersome mixing technology would then be needed in the print head to fairly mix the components properly during the printing process.
These disadvantages of the use of a two-component UV ink jet printing ink create a great demand for an improved UV ink jet printing ink.
Another disadvantage associated with the photo curable ink composition of the aforementioned prior art documents is that especially the monofunctional monomers are smelling and vaporize before and during the curing or hardening of the ink compositions. The monofunctional monomeric compounds are frequently toxic and hazardous for a user, such as the staff of a print shop or an office.
There is a demand to use effect pigments in printing inks to be able to produce print images having superior optical properties, e.g. having a metallic or optical variable impression. Effect pigments produce an improved optical appearance with increased particle size. Unfortunately, effect pigments tend to settle in ink jet printing ink compositions due to gravity. If the ink jet printing ink composition is a paint or lacquer, the ink jet printing ink composition can easily be mechanically stirred to disperse the effect pigments. When using effect pigments in a printer cartridge, stirring of the ink would be very cumbersome.